The present invention relates to a maintenance technology of an incore piping section of a reactor such as a boiling water reactor or the like, and in particular, to an incore piping section maintenance system of a reactor, which performs preventive repair and preventive maintenance of weld (welded or to be welded) zones or portions in an incore piping section located in a reactor pressure vessel.
A boiling water reactor such as a light water reactor is constructed as shown in a longitudinal cross-sectional view of FIG. 7. A reactor core 2 is installed in a reactor pressure vessel 1, and the reactor core 2 is immersed in a coolant 3. Further, the reactor core 2 is constructed in a manner that a plurality of fuel assemblies (not shown) and control rods are arranged in a cylindrical core shroud 4.
A reactor water (coolant) 3 in the reactor pressure vessel 1 flows upward through the core 2 from a core lower plenum 9. The coolant 3 receives a nuclear reaction energy when flowing upward through the core 2, and then, its temperature and pressure rise up, and thus, becomes a two-phase flow state of water and steam (vapor). The coolant 3, which is in a gas-liquid two-phase flow state, flows into a steam separator 5 located above the reactor core 2, and then, is separated into water and steam by means of the steam separator 5. A steam thus gas-liquid separated is introduced into a steam desiccator or drier 6 located above the steam separator 5, and then, is dried here so as to become a dry steam. The dry steam is supplied as a main steam to a steam turbine (not shown) via a main steam pipe (tube) 7 connected to the reactor pressure vessel 1, and then, is used for power generation.
On the other hand, a water thus gas-liquid separated is guided to a truss or sleeve-like downcomer portion 8 between the reactor core 2 and the reactor pressure vessel 1, and then, flows downward through the downcomer portion 8, and thus, is guided to a core lower plenum 9. Further, in the downcomer portion 8, an outer periphery of the core shroud 4 is provided with a plurality of jet pumps 10 at equal intervals.
Meanwhile, the core lower plenum 9 below the reactor core 2 is provided with a control rod guide pipe 11, and a control rod driving mechanism 12 is located below the control rod guide pipe 11. The control rod driving mechanism 12 carries out a control for inserting and pulling a control rod into and out of the reactor core 2 through the control rod guide pipe 11, and thus, performs a power control of reactor.
Moreover, two reactor re-circulation systems including a reactor re-circulation pump (not shown) are located outside the reactor pressure vessel 1. When the re- circulation pump of the reactor re-circulation system is operated, a coolant in the reactor pressure vessel 1 passes through a reactor re-circulation system (not shown) from a cooler re-circulation water outlet nozzle 14, and then, is returned into the reactor pressure vessel 1, and thus, is guided to the jet pump 10 via the re-circulation water inlet nozzle 13. The jet pump 10 sucks its surrounding coolant, and then, supplies it into the core lower plenum 9. More specifically, by a driving water supplied from the reactor re-circulation pump to the jet pump 10, the jet pump 10 forcibly circulates the coolant 3 in the reactor core 2 via the core lower plenum 9.
On the other hand, the reactor pressure vessel 1 is provided with a core spray system 15 which constitutes an emergency cooling system of a reactor. The core spray system 15 has a piping arrangement as shown in FIG. 5 and FIG. 6. FIG. 6 is a perspective view showing a state that the core spray system 15 is located in the reactor pressure vessel.
As shown in FIG. 5, the core spray system 15 extends into the core shroud 4 from the outside of the reactor pressure vessel 1 penetrating through the reactor pressure vessel (RPV) 1 and the core shroud and includes a core spray system pipe for introducing a spray water into the core shroud 4. The core spray system pipe is a piping part for connecting the RPV 1 and the core shroud 4 in the RPV 1.
Moreover, a pipeline of the core spray system 15 is arranged as shown in FIG. 6. In the core spray system 15, an incore branch part 16 is connected to one end of the core spray system pipeline after penetrating through the RPV 1. A semi-circular pipe 17 is formed in a manner of extending from the incore branch part 16 like a semi-circular arc and branching right and left. Each end portion of the semi-circular pipe 17 branching right and left is formed at a position separating by an angle of about 180xc2x0 along an inner wall of the RPV 1. The semi-circular pipe 17 is connected with a vertical pipe 18 which extends downward from each end portion thereof. A lower end of the vertical pipe 18 constitutes the other end of the core spray system pipeline. A lower end of each vertical pipe 18 is connected via a sleeve 20 to a riser pipe 19 which rises up from the core shroud 4, and thus, a core spray system pipeline is constructed. The core spray system pipeline functions as a reactor emergency cooling system into which a cooling water for cooling the core is supplied in a reactor emergency shutdown. When the emergency cooling system is operated, a fluid vibration, thermal deformation and the like are generated in the core spray system pipeline.
For this reason, the core spray system pipeline is used under severe circumstances as compared with other equipments, and as a result, a great load is applied to each member of the core spray pipeline, and as the case may be, a great stress is applied to the core spray pipe.
Some early nuclear power plants have been operated for more than twenty years, and hence, stable operation for aged plants makes it more vitally important to implement the preventive maintenance of a reactor pressure vessel and internal elements of the early plants which were made of high carbon stainless steel susceptible to Stress Corrosion Cracking (SCC). As mentioned hereinlater, the SCC is caused by the combination of three factors of Material, Stress and Environment, and it is important to get rid of one of three factors for the preventive maintenance.
In the event that an excessive load is applied to the core spray pipe of the core spray system 15 due to any factors, or an inner surface of the core spray pipe rusts away, there is the possibility that a crack or the like is generated in the pipe due to the rust.
Furthermore, because an austenitic stainless steel pipe is mainly used as a material for the core spray pipe, if the following three factors, that is, Stress, Corrosion Environment and Material (generation of chromium deficiency layer) are realized, the Stress Corrosion Cracking (SCC) is generated, and for this reason, it is anticipated that the core spray pipe is damaged.
This stress corrosion cracking phenomenon does not happen if any one of the three factors, mentioned above, lacks. In order to prevent this stress corrosion cracking, there is a need of making various measures so that the aforesaid three factors are not established. Moreover, in the case where a rush and crack is generated in a surface of the core spray pipe due to any factors, when these rush and crack have left, the crack is progressing, and as a result, there may be the case where a crack is generated in the core spray pipe. Thus, when the core spray system 15, which functions as an emergency cooling system of a reactor, becomes a state as described above, it is anticipated that a harmful influence is given to other equipments included in the core, thus being not preferable.
Furthermore, recently, a laser de-sensitization treatment (LDT) technology has mainly been developed for the preventive maintenance of the thin pipe and plate. A high power laser beam produces a molten layer and solution heat treated layer and can change the sensitized surface of a stainless steel to be de-sensitized.
The LDT is a treatment for suppressing a sensitivity (de-sensitization) of an lntergranular Stress Corrosion Cracking (IGSCC) by the steps of irradiating with laser beams a surface of a stainless steel sensitized by an influence of welding heat or the like and forming a solution heat treatment layer and a molten coagulation layer.
That is, FIG. 8 shows relationship among the above mentioned three factors such as Material, Stress and Environment for improving the SCC proof property in view of the de-sensitization treatment, and as shown in FIG. 9, when a YAG laser beam of high energy density passing through an optical fiber, for example, is irradiated on a laser execution portion through optical means such as mirror or lens, the portion subjected to the laser execution is rapidly heated, a Cr carbide is decomposed and, hence, a Cr-lacking layer near a grain boundary is lost. After the laser beam has passed, the laser execution portion is rapidly cooled and the surface thereof is de-sensitized. By continuously performing such de-sensitization treatment to the surface contacting the solution, the solution heat treatment layer and the molten coagulation layer are formed.
The present invention has been made in view of the above-mentioned circumstances. It is, therefore, an object of the present invention to provide an incore piping section maintenance system of a reactor, which can securely and effectively perform a preventive maintenance treatment such as a surface de-sensitization of the reactor incore piping section by a laser beam irradiation, that is, a laser de-sensitization treatment, and thus, can improve normalization, soundness and reliability of the reactor core piping section.
Another object of the present invention is to provide an incore piping section maintenance system of a reactor, which can carry out a laser irradiation through remote control with respect to a maintenance target portion of the reactor core piping section so as to perform a surface de-sensitization of weld zone, that is, a laser de-sensitization treatment for a short time, and can effectively and smoothly perform a preventive maintenance such as a preventive repair or the like.
Still another object of the present invention is to provide an incore piping section maintenance system of a reactor, which can effectively perform an inspection, repair or polishing work of the reactor incore piping section through remote control without draining off a reactor water in a reactor pressure vessel.
These and other objects can be achieved, according to the present invention, by providing, in one aspect, an incore piping section maintenance system of a reactor, comprising:
a maintenance system main body which is fixed to a maintenance target portion in a reactor pressure vessel or in the vicinity thereof to which a preventive-maintenance operation is executed;
support means provided for the maintenance system main body so as to be movable in a reciprocal manner towards the maintenance target portion;
laser generation means for generating a laser beam;
laser de-sensitization treatment means which is rotatably supported around an axis of the support means and which includes a laser irradiation section for irradiating the laser beam to the maintenance target portion; and
optical transmission means which guides the laser beam outputted from the laser generation means to the laser de-sensitization treatment means.
In this aspect, the maintenance target portion is an incore piping section located in the reactor pressure vessel, the support means includes seal means including expandable seal members so as to seal both sides of the laser irradiation section, and the seal means forms an atmospheric environment to the laser irradiation section so that the laser irradiation section between the seal members is filled with a purge gas, a laser irradiation being then carried out in the purge gas.
The laser de-sensitization treatment means further includes an inspection monitoring means provided for the laser irradiation section or in the vicinity thereof. The laser de-sensitization treatment means further includes a maintenance target portion detector which detects and confirms a laser execution position to which the preventive-maintenance operation is executed. The maintenance target portion detector is an ultrasonic flaw detector which detects and confirms the laser execution position. The maintenance target portion detector may be a ferrite indicator, the ferrite indicator distinguishing a difference in ferrite quantity between a weld zone and a base material of the incore piping section so as to detect and confirm a laser execution position.
The laser de-sensitization treatment means further includes a polishing means so that the laser execution portion is subjected to polishing working by means of the polishing means.
In another aspect, there is provided an incore piping section maintenance system of a reactor, comprising:
a maintenance system main body to be inserted into a pipe of an incore piping section located in a reactor pressure vessel;
main body supporting means for detachably fixing the maintenance system main body in the pipe;
a turning arm supported to the maintenance system main body;
turning means turning and driving the turning arm;
axial moving means which is supported so as to be slidable in a direction substantially perpendicular to the turning arm, the axial moving means being movable in an axial direction with respect to a header;
laser generation means for generating laser beam;
laser de-sensitization treatment means which is supported on the axial moving means and includes a laser irradiation section for irradiating the laser beam to an outer surface of the pipe; and
optical transmission means which guides a laser beam outputted from the laser generation means to the laser de-sensitization treatment means.
In this aspect, the main body supporting means includes at least three main body supporting mechanisms and each of the main body supporting mechanisms is constructed in combination with a link mechanism including a guide member and a cylinder apparatus for driving the guide member of the link mechanism so that the guide member comes in and out from the maintenance system main body.
According to the present invention of the characters mentioned above, in the incore piping section maintenance system of a reactor, the laser de-sensitization treatment means is located in the pipe of the incore piping section or on a predetermined position on the pipe outer peripheral surface, and a laser beam is irradiated from the laser de-sensitization treatment means to the entire periphery of the incore piping section thereby to perform a surface de-sensitization of the incore piping section and a laser de-sensitization treatment and to effectively perform preventive repair and preventive maintenance by a laser beam. Therefore, it is possible to greatly enhance normalization, soundness and reliability of the incore piping section such as a core spray pipe or the like.
Further, in the incore piping section maintenance system of a reactor according to the present invention, it is possible to carry out a surface de-sensitization, that is, laser de-sensitization treatment through the laser de-sensitization treatment means in the water by remote control. Therefore, a maintenance work can be readily performed, and also, it is possible to greatly reduce a possibility that a worker is exposed to a radiation.
Furthermore, in the incore piping section maintenance system of the present invention, it is possible to stably set the maintenance system main body onto the maintenance target portion of the incore piping section or in the vicinity thereof by the remote control. Therefore, a work for the preventive maintenance and the preventive repair of the incore piping section can be stably and effectively carried out.
The nature and further characteristic features of the present invention will be made more clear from the following descriptions made with reference to the accompanying drawings.